1. Field of the Invention
The present disclosure generally relates to a booster circuit for boosting a voltage fed to an induction-heating unit for heating an object, and more particularly to a power-supply unit including a booster circuit, and an image forming apparatus including a power-supply unit having a booster circuit.
2. Discussion of the Background
In general, an induction-heating unit (IH unit) may be driven by a commercial power supply such as alternating current (AC).
Such an IH unit may have a protection function that may protect the IH unit from abnormal factors such as a voltage surge due to a lightning strike, for example, momentary power failure, sudden voltage decrease, and sudden voltage increase.
Hereinafter, the induction-heating unit will be termed an “IH unit” for the simplicity of expression, as required.
The IH unit may have a voltage-resonant circuit, which may generate a voltage having a value obtained by multiplying an input voltage by a “Q factor”. “Q factor” means “quality factor” (hereinafter referred as “Q factor,” as required).
The “Q factor” may be used as an indicator to indicate a performance level of a voltage-resonant circuit.
The “Q factor” can be computed as below with following equations of (1) and (2) with following settings:    frequency of voltage-resonant circuit of “ω0”; coil    inductance of “L”; capacitance of capacitor of “C”; and    equivalent resistance value of “R” for a circuit.Q=ω0L/R  (1)Q=(1/R)×(1/ω0C)  (2)
For example, in case of a series resonance circuit, a voltage of a coil or capacitor may become a voltage value, which may be obtained by multiplying an input voltage, supplied by a power source, by a “Q factor.”
A circuit may have an electronic switch, which may be used for switching a relatively greater power (or electricity).
For example, such an electronic switch may include an “insulated gate bipolar transistor (IGBT).” Hereinafter, the “insulated gate bipolar transistor” may be termed “IGBT” for the simplicity of expression.
FIG. 1 shows an example circuitry of an IGBT, and FIG. 2 shows an example output waveform of each terminal of the IGBT when an electric power of 1,200 W(watt) is input to the IGBT for induction-heating.
The IGBT may be a bipolar transistor, which may include a MOSFET (metal oxide semiconductor field effect transistor) 110 at a gate portion of the IGBT, and has a gate terminal G, a collector terminal C, and an emitter terminal E as shown in FIG. 1.
Such an IGBT can be driven by applying a voltage between the gate terminal G and emitter terminal E, and may have a function of arc-suppressing, in which an ON/OFF switching can be conducted by an input signal. Such an IGBT may be a solid-state device, which can switch a greater power (or electricity).
As shown in FIG. 2, when an input voltage 111 is input to the IGBT, a gate voltage 112 at the gate terminal G and a collector voltage 113 at the collector terminal C may change.
Such an IGBT can switch a relatively greater power (or electricity) compared to a FET (field-effect transistor), but a switching speed of the IGBT may be relatively slower than a switching speed of the FET.
An image forming apparatus may employ an induction-heating unit (IH unit) for fixing an image on a sheet, in which a switching operation may be conducted at a greater power (e.g., dielectric strength of 1,000V and electric current 60 A).
Such switching at greater power cannot be conducted by a FET, which may be used for a normal level of power supply switching. Accordingly, an IGBT may be used for such switching at a greater power (or electricity).
Conventionally, an IH unit may reduce an induction heating time by increasing an induced electromotive force (or electricity).
Increasing a resonance voltage may increase such an induced electromotive force.
A peak value of a resonance voltage (referred to as “resonance peak value”) may be increased by changing an inductance component or capacitor component of the IH unit. Such a “resonance peak value” can be increased by reducing a resonance time, in general.
However, an IGBT used as an electronic switch for induction-heating may have an upper limit for switching speed, and thereby, a conventional circuit having the IGBT may not be preferable from a viewpoint of increasing switching speed.
Furthermore, if a switching speed may be increased forcedly in such a conventional circuit, a switching loss of the IGBT may become unfavorably greater.
Furthermore, if a resonance peak value (or Q factor) is increased, a peak value of a waveform may become greater. In such a condition, the Q factor may fluctuate even if a frequency change may occur in a smaller level, which may not be favorable for controlling the IGBT.
FIGS. 3A to 3C show output waveforms of each terminal of an IGBT used for an induction-heating operation.
FIG. 3A shows a condition that a resonance period becomes longer and a resonance is not realized, in which a noise may be generated in a superimposed portion, and a greater loss occurs.
FIG. 3B shows a condition that a resonance is realized, in which an IGBT may be efficiently driven.
FIG. 3C shows a condition that a resonance may be realized, but a resonance period may be shorter and a peak value of the waveform may become higher, in which controlling the IGBT may become difficult. Furthermore, a greater voltage may be applied to a coil, by which a greater heat may be generated, wherein such heat generation may result in a greater loss. Then, an electric current may flow to a body diode of the IGBT, and a greater loss may be observed for a circuit.
As such, in a conventional IH unit, a Q factor may be increased to increase an electromotive force so that a heating speed rate may be increased. In such a condition, a peak value of a resonance waveform may become higher, by which controlling the IGBT may become difficult.
FIG. 4 shows an example block diagram explaining a functional configuration of an induction-heating unit 90.
The induction-heating unit 90 may include an IH cooking heater, for example. Hereinafter, the induction-heating unit 90 may be termed as “IH unit 90” for simplicity of expressions.
The IH unit 90 may have a top plate (not shown) and a heating coil 94 placed under the top plate.
The heating coil 94 may heat a cooking pan 95, which is used as an object to be heated by the IH unit 90. The cooking pan 95 may be made of a metal such as iron, aluminum, stainless steel, or the like.
Such an IH unit 90 may heat the cooking pan 95, which may contain material such as water with an induction-heating method. By heating the cooking pan 95 as such, water in the cooking pan 95 may be warmed or heated.
Furthermore, the IH unit 90 may include a commercial power supply 91, a rectifier 92, and an inverter 93, for example.
When a power supply to the IH unit 90 is set to an ON condition, an alternating current (AC) may flow on the heating coil 94, which may be placed under the top plate.
Such an alternating current (AC) may be a higher frequency wave having a given frequency (e.g., 20 KHz). Such a higher frequency wave of alternating current (AC) may be generated from a direct current by the inverter 93 as below.
For example, the commercial power supply 91 may supply alternating current (AC) having a given frequency and voltage (e.g., 60 Hz or 50 Hz and AC 100V) to the rectifier 92. The rectifier 92 may rectify the alternating current (AC) to direct current (DC), and supply the direct current (DC) to the inverter 93.
The inverter 93 may invert the direct current (DC) to an alternating current (AC) having a higher frequency wave, and may flow the alternating current (AC) to the heating coil 94.
When such an alternating current (AC) may flow in the heating coil 94, a magnetic field may be generated around the heating coil 94.
Such a magnetic field may induce an electric current called an “eddy current” on the cooking pan 95 placed over the heating coil 94.
If a direct current (DC) flows in the heating coil 94, such an eddy current may be generated to the cooking pan 95 for a moment when a DC power supply is set to ON.
The eddy current may result into a heat energy measured as joule heat, which may be energy loss. Another energy loss such as hysteresis loss may occur but such energy loss may be practically ignored.
Such an eddy current may have a flow direction, which may be opposite to a flow direction of an electric current flowing in the heating coil 94.
The eddy current may generate heat energy in an object (e.g., the bottom of the cooking pan 95) to heat the cooking pan 95 with the heat energy. Accordingly, an object (e.g., the bottom of the cooking pan 95) may be directly heated by an induction heating method.
A heating value “W” for such induction heating can be computed as below.W=I2×R wherein “I” represents an eddy current, and “R” represents an electrical resistivity of the bottom of the cooking pan 95.
If the cooking pan 95 has water therein, the cooking pan 95 heated by such heat energy may transfer the heat energy to water in the cooking pan 95, by which water in the cooking pan 95 may be warmed or heated to hot water.
Furthermore, such an induction heating unit may be employed for an office automation (OA) apparatus.
A conventional image forming apparatus (e.g., copier) may employ a halogen heater for a toner fixing process.
However, a recently marketed image forming apparatus may have employed the above-explained induction heating unit, by which a temperature control for a toner fixing process may be more precisely conducted, and a warming-up time may be shortened, and thereby, such an induction heating unit may be effective for reducing energy consumption of an image forming apparatus.
FIG. 5 shows a block diagram of an IH unit 1A having a conventional configuration.
The IH unit 1A may be operated as below when conducting an induction-heating operation.
(1) A commercial power supply 4 may supply an AC 100V (as a commercial voltage) to a rectifying circuit 2, and the rectifying circuit 2 directly rectifies AC 100V to DC 141V.
(2) An inverter circuit 3A having an induction-heating (IH) controller 6 (as a microcomputer) and a drive circuit 7 may convert the DC 141V to a higher frequency wave having 600V0-p, 50 A0-p, and 20 KHz to 40 KHz.
(3) The inverter circuit 3A may include an IGBT 5 as a switching device (or element), which can conduct a switching operation for a greater power (or electricity).
(4) The IH controller 6 may control an ON/OFF operation of the IGBT 5 with the drive circuit 7.
(5) The above-mentioned operation at (4) may be a voltage resonance operation.
(6) The inverter circuit 3A may include a diode D1 as a body diode for the IGBT 5.
(7) The IH controller 6 may control an induction heating operation, and also control a resonance point tracking, electric current protection, and voltage protection.
Hereinafter, a physical phenomenon of induction heating is explained with reference to FIG. 6, which shows the fundamentals of induction heating.
(1) When a power supply 101 supplies alternating current (AC) to a coil 100, an electric current may flow in the coil 100, and the current may generate a magnetic field MF around the coil 100.
(2) Such a magnetic field MF may also exist around a metal cylinder 102 used as an electric conductor, which is an object placed inside the coil 100.
(3) Then, an electric current called an “eddy current EC” may flow in the metal cylinder 102 in a given direction to cancel an effect of the magnetic field MF. The eddy current EC may flow in a sub-surface portion, having a depth δ, of the metal cylinder 102.
In general, an electric current density may become greater as the electric current gets closer to a surface of an electric conductor (e.g., metal cylinder 102) and may become smaller as the electric current gets further away from the surface of an electric conductor, wherein such a phenomenon may be called as “skin effect.”
The higher the frequency of the electric current, the higher the electric current density at the surface, and a higher electric current density may increase an impedance of an electric conductor.
(4) The eddy current EC and an electric resistivity of the metal cylinder 102 may generate a joule heat in the metal cylinder 102. Because the metal cylinder 102 may have more electric current in its surface portion, the surface of the metal cylinder 102 may be heated to a greater level.
(5) With such a process, a temperature on the surface of the metal cylinder 102 may be increased, and also heat dissipation from the metal cylinder 102 may occur concurrently.
(6) A heat transfer may occur from the surface to core portion of metal cylinder 102, by which the core portion of metal cylinder 102 may be heated after the surface of metal cylinder 102 is heated.